Under circumstances where measurements must be made on a fluid that is toxic or hazardous to human health, care must be exercised to prevent even small quantities of the fluid from escaping during the insertion, removal or replacement of a sensing element. In processes that use carcinogenic compounds, for example, the release of even tiny amounts of such compounds into the work environment during the course of the insertion and/or removal of a sensing element, such as a conductivity probe, can result in a significant long term health hazard for the workers who operate the process. The need for a more nearly completely leak-proof sensor mechanism of this type has become increasingly urgent as the list of compounds known to be hazardous to human health has grown with advances in medical knowledge.
One approach to solving this problem has involved the avoidance of direct contact between the sensing element and the sample fluid. An example of this approach is shown in U.S. Pat. No. 3,246,521 issued in the name of L. A. Humphrey on Apr. 19, 1966. In the referenced patent, the sensing element is a thermometer that is exposed to the sample fluid only through a protective jacket. When the thermometer is to be removed or replaced, the jacket is withdrawn into the interior of the sensor mechanism and sealed off from the environment by means of a suitable valve.
One disadvantage of the above described structure is that it provides no way of preventing the escape of the residual quantities of sample fluid that are trapped in the interior of the mechanism after the closure of the valve. Instead, the small interior volume of the mechanism is relied upon to assure that the amount of the sample fluid that escapes is small. Recent advances in medical science, however, indicate that even extremely small quantities of hazardous substances can, over a period of prolonged exposure, give rise to serious health problems. Thus, prior to the present invention, sensor mechanisms of the above type were not sufficiently effective in eliminating the problem of human exposure.
Another approach to dealing with the problem of preventing hazardous substances from entering the human environment is described in U.S. Pat. No. 2,887,443 issued in the name of R. D. Blue et al. on May 19, 1959. In devices of the latter type, an electrode used in an electrolytic cell is withdrawn from the contaminated interior atmosphere of the cell into a side chamber having sliding metal doors that are intended to close the chamber off from the electrolytic cell. The interior of the chamber is then purged by pumping clean air into the chamber while allowing the contaminating gas to escape through a purge outlet duct. The electrode was then removed by loosening the bolts that sealed the top plate of the side chamber to the body thereof.
One problem with the above described structure was that the abutting seal provided by the meeting of the sliding metal doors was only partly effective. Moreover, the seal between the sliding doors and the structure that supported them provided an additional avenue for escape of noxious gases. As a result, this structure provided protection only against the large scale escape of gas from the electrolytic cell.
Another shortcoming of the above-described structure is that the removal and replacement of the electrode element was a laborious process requiring the loosening and tightening of numerous bolts. In addition, the above structure has a number of sliding seals and gaskets the deterioration of which could result in further escape of noxious gases. This problem was especially severe because of the extremely high temperature and highly corrosive environment of the electrolytic cell.